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Abstract:

The invention relates to a method for low-temperature
cooling/liquefaction of a working fluid, in particular a working fluid
including helium or consisting of pure helium by means of a
refrigerator/liquefier that includes a working circuit provided with a
compressor station and a cold box. The refrigerator/liquefier subjects
the working gas within the working circuit to a cycle that includes, in
series: compressing the working fluid within the compressor station,
cooling and decompressing the working fluid in the cold box, and heating
the working fluid with a view to the return thereof to the compressor
station. The compressor station includes one or more compression levels,
each one using one or more compressors mounted on landings. The method is
characterized in that the refrigerator includes a device for injecting a
seal gas that is separate from the working fluid on at least one landing
of the compressor(s) so as to form a gas seal that guides the working
fluid leaks, coming from the working circuit, to an area for
recirculating and returning the fluid into the working circuit.

Claims:

1-15. (canceled)

16. A method for low-temperature cooling/liquefaction of a working fluid
using a refrigerator/liquefier comprising a working circuit provided with
a compression station and with a cold box, comprising the step of, using
the refrigerator/liquefier, subjecting the working gas in the working
circuit to a cycle comprising in series: a compression of the working
fluid in the compression station, the compression station comprising one
or several compression stages each using one or several compressors
mounted on bearings; a cooling and an expansion of the working fluid in
the cold box; and a reheating of the working fluid so that it can be
returned to the compression station, wherein the refrigerator/liquefier
comprises a barrier gas injection device adapted to inject a barrier gas,
that is distinct from the working fluid, into at least one bearing of the
compressor(s) to form a gaseous barrier guiding leaks of the working
fluid originating from the working circuit towards a zone for recycling
and returning them to the working circuit.

17. The method of claim 16, wherein the barrier gas injection device
forms a gaseous barrier prevent the leaks of working fluid from passing
towards at least one contaminated zone of the compression station, the
contaminated zone being an oil-containing mechanism of the compression
station or being a zone which is not sealed with respect to the
atmosphere.

18. The method of claim 16, wherein the barrier gas is injected into said
at least one bearing at a pressure lower than a pressure of the working
fluid in the working circuit at the compressor that is mounted on said
bearing.

21. The method of claim 16, wherein the barrier gas injection device
comprises at least one barrier gas injection point and at least one
outlet adapted to collect the mixture of the barrier gas and the leaked
working fluid.

22. The method of claim 21, wherein: the refrigerator/liquefier comprises
a gas purification member having an inlet adapted to receive gas that is
to be purified and an outlet for purified gas, the outlet of the
purification member being fluidically connected to the working circuit at
an outlet of the compression station; and at least part of the mixture of
barrier gas and working fluid collected by the at least one outlet is
reinjected into the inlet of the purification member so that the mixture
can be purified and then reinjected into the working circuit at the
outlet of the compression station.

23. The method of claim 22, wherein the inlet of the purification member
is fed with gas by a gas that is distinct from the working fluid of the
circuit, that is to say that the working circuit is of the "open" type.

24. The method of claim 21, wherein at least part of the mixture of
barrier gas and of working fluid collected by the at least one outlet of
the barrier gas injection device is reinjected into the working circuit
at a position selected from the group consisting of: an inlet of the
compression station, an intermediate compression stage of the compression
station, an outlet of the compression station, and combinations thereof.

25. The method of claim 24, wherein the cycle is a closed cycle in that
it comprises a gas purification member having an inlet that is fed only
with working gas originating from the working circuit and an outlet which
feeds the cold box.

26. The method of claim 21, wherein: the refrigerator/liquefier comprises
a gas purification member having an inlet for gas that is to be purified
and an outlet for purified gas, the outlet of the purification member
being fluidically connected to the working circuit at an inlet to the
compression station; and at least part of a mixture of barrier gas and of
working fluid collected by the at least one outlet of the barrier gas
injection device is reinjected into the inlet of the purification member
so that it can be purified and then reinjected into the working circuit
at an inlet of the compression station.

27. The method of claim 21, wherein: the refrigerator/liquefier comprises
a gas purification member having an inlet for gas that is to be purified
and an outlet for purified gas, the outlet of the purification member
being fluidically connected to the working circuit at an intermediate
compression stage of the compression station and/or at an outlet of the
compression station; and at least part of a mixture of barrier gas and of
working fluid collected by the at least one outlet of the barrier gas
injection device is reinjected into the inlet of the purification member
so that it can be purified and then reinjected into the working circuit
in or at an outlet of the compression station.

28. The method of claim 27, wherein at least part of the mixture of
barrier gas and of working fluid is compressed and fed to the inlet of
the purification member after said compression.

29. The method of claim 16, wherein the working fluid comprises helium.

30. The method of claim 16, wherein the working fluid consists of pure
helium.

31. A device for the low-temperature cooling/liquefaction of a working
fluid containing helium or consisting of pure helium, the device
comprising a working circuit provided with a compression station and with
a cold box, the working circuit subjecting the working gas to a cycle
comprising, in series: a compression of the working fluid in the
compression station, the compression station comprising one or several
compression stages each using one or several compressors mounted on
bearings; a cooling and an expansion of the working fluid in the cold
box; and a reheating of the working fluid so that it can be returned to
the compression station, wherein the device for the low-temperature
cooling/liquefaction comprises a barrier gas injection device adapted to
inject a barrier gas, that is distinct from the working fluid, into at
least one of the bearings to form a gaseous barrier guiding leaks of
working fluid originating from the working circuit towards a zone for
recycling and returning them to the working circuit.

32. The device of claim 31, wherein: the barrier gas injection device
comprises at least one barrier gas injection point and at least one
outlet adapted to collect a mixture of injected barrier gas and leaked
working fluid; and the circuit comprises a pipe reinjecting said mixture
into the working circuit at a point selected from the group consisting of
an inlet of the compression station, an intermediate compression stage of
the compression station, at the outlet of the compression station, and
combinations thereof.

33. The device of claim 32, wherein the low-temperature
cooling/liquefaction device further comprises a member adapted to:
purifying the mixture so as to separate impurities, including the barrier
gas, from the working gas; and reinject the purified mixture into the
working circuit once it has passed through the purification member.

Description:

[0001] The present invention relates to a low-temperature
cooling/liquefaction method and device.

[0002] The invention may notably relate to a liquefaction method and
device and to a refrigeration method and device operating on helium.

[0003] The invention relates more specifically to a method for the
low-temperature cooling/liquefaction of a working fluid, notably a
working fluid containing helium or consisting of pure helium using a
refrigerator/liquefier comprising a working circuit provided with a
compression station and with a cold box, the refrigerator/liquefier
subjecting the working gas in the working circuit to a cycle comprising
in series: a compression of the working fluid in the compression station,
a cooling and an expansion of the working fluid in the cold box and a
reheating of the working fluid so that it can be returned to the
compression station, the compression station comprising one or several
compression stages each using one or several compressors mounted on
bearings.

[0004] Refrigerators or liquefiers operating at low temperature (for
example below 80K or below 20K) conventionally use a working fluid (for
example helium) that is subjected to a working cycle comprising a
compression, an expansion, a cooling and a reheating. These apparatuses
in general require several stages for compressing the working gas. Each
compression stage uses one or more compressor impellers. One example is a
compressor of the centrifugal type.

[0005] Leaks of working gas in the compression station at the interface
between the rotating parts and the fixed parts are unavoidable.
Particularly when the working gas is helium, relatively significant leaks
of gas are observed at the bearings that support the shafts of the
compressor impellers. In order to limit this loss of relatively expensive
working gas, it is known practice for the leakage at each bearing of each
compression stage to be limited using members such as packings that form
labyrinths for the gas, oil seals, floating ring seals, gas seals, etc.

[0006] Aside from the fact that these devices increase the cost of the
installation, these known systems are not always best suited to
cooler/liquefier technologies.

[0007] In addition, the oil present in the mechanism of the compression
station must not be allowed to contaminate the working gas (by mixing
with the helium or by adding moisture and/or light hydrocarbons). This is
because such impurities introduced into the working circuit could create
blockages at cryogenic temperatures and cause equipment breakage.

[0008] It is an object of the present invention to alleviate all or some
of the abovementioned disadvantages of the prior art.

[0009] To this end, the method according to the invention, in other
respects in accordance with the generic definition thereof given in the
above preamble, is essentially characterized in that the refrigerator
comprises a device for injecting a barrier gas distinct from the working
fluid into at least one bearing of the compressor or compressors to form
a gaseous barrier guiding the leaks of working fluid originating from the
working circuit towards a zone for recycling and returning them to the
working circuit.

[0010] Moreover, some embodiments of the invention may include one or
several of the following features: [0011] the device for injecting the
barrier gas forms a gaseous barrier to prevent the leaks of working fluid
from passing towards at least one so-called contaminated zone of the
compression station which zone is an oil-containing mechanism of the
compression station or a zone which is not sealed with respect to the
atmosphere, [0012] the barrier gas is injected into at least one bearing
and at a pressure lower than the pressure of the working fluid in the
working circuit at the compressor mounted on the said bearing, [0013] the
barrier gas contains nitrogen or consists of pure nitrogen, [0014] the
device for injecting the barrier gas comprises at least one barrier gas
injection point and at least one outlet intended for collecting the
mixture containing the injected barrier gas and the working fluid
originating from the leak or leaks, [0015] the refrigerator/liquefier
comprises a gas purification member having an inlet for gas that is to be
purified and an outlet for purified gas, the outlet of the purification
member being fluidically connected to the working circuit at the outlet
of the compression station, and at least part of the mixture of barrier
gas and of working fluid collected by the at least one outlet is
reinjected into the inlet of the purification member so that it can be
purified and then reinjected into the working circuit at the outlet of
the compression station, [0016] the inlet of the purification member is
fed with gas by a gas that is distinct from the working fluid of the
circuit, that is to say that the working circuit is of the "open" type,
[0017] at least part of the mixture of barrier gas and of working fluid
collected by the at least one outlet is reinjected into the working
circuit at the inlet of the compression station and/or at an intermediate
compression stage and/or at the outlet of the compression station, [0018]
the working cycle is said to be a "closed" cycle and comprises a gas
purification member having an inlet for gas that is to be purified fed
only with working gas originating from the working circuit, and an outlet
for purified gas which feeds the cold box, [0019] the
refrigerator/liquefier comprises a gas purification member having an
inlet for gas that is to be purified and an outlet for purified gas, the
outlet of the purification member being fluidically connected to the
working circuit at the inlet to the compression station, and at least
part of the mixture of barrier gas and of working fluid collected by the
at least one outlet is reinjected into the inlet of the purification
member so that it can be purified and then reinjected into the working
circuit at the inlet of the compression station, [0020] the
refrigerator/liquefier comprises a gas purification member having an
inlet for gas that is to be purified and an outlet for purified gas, the
outlet of the purification member being fluidically connected to the
working circuit at an intermediate compression stage of the compression
station and/or at the outlet of the compression station, and at least
part of the mixture of barrier gas and of working fluid collected by the
at least one outlet is reinjected into the inlet of the purification
member so that it can be purified and then reinjected into the working
circuit in or at the outlet of the compression station, [0021] at least
part of the mixture of barrier gas and of working fluid collected by the
at least one outlet is compressed before it is fed to the inlet of the
purification member, [0022] the device comprises a member for purifying
the mixture so as to separate impurities from the working gas and notably
so as to remove the barrier gas from the mixture, the mixture being
reinjected into the working circuit once it has passed through the
purification member, [0023] the purification member comprises a
separation system to remove impurities other than the working fluid such
as the nitrogen from the gas, [0024] the purification member possibly
comprises a system for compressing the purified gas or gas that is to be
purified, [0025] the compressor or compressors are of the centrifugal
type, [0026] the expansion turbine or turbines of the cold box are of the
centrifugal type.

[0027] The invention also relates to a device for the low-temperature
cooling/liquefaction of a working fluid containing helium or consisting
of pure helium, the device comprising a working circuit provided with a
compression station and with a cold box, the working circuit subjecting
the working gas to a cycle comprising, in series: a compression of the
working fluid in the compression station, a cooling and an expansion of
the working fluid in the cold box and a reheating of the working fluid so
that it can be returned to the compression station, the compression
station comprising one or several compression stages each using one or
several compressors mounted on bearings, characterized in that the
refrigerator comprises a device for injecting a barrier gas distinct from
the working fluid into at least one bearing of the compressor or
compressors to form a gaseous barrier guiding the leaks of working fluid
originating from the working circuit towards a zone for recycling and
returning them to the working circuit.

[0028] According to other possible particular features:

[0029] the device for injecting the barrier gas comprises at least one
barrier gas injection point and at least one outlet intended for
collecting the mixture of injected barrier gas and working fluid
originating from the leak or leaks, and the circuit comprises a pipe
reinjecting the said mixture into the working circuit at the inlet of the
compression station and/or at an intermediate compression stage of the
compression station and/or at the outlet of the compression station;

[0030] the device comprises a member for purifying the mixture so as to
separate impurities from the working gas and notably so as to remove the
barrier gas from the mixture, the mixture being reinjected into the
working circuit once it has passed through the purification member.

[0031] The invention may also relate to any alternative method or device
comprising any combination of the features listed hereinabove or
hereinbelow.

[0032] Other specifics and advantages will become apparent from reading
the following description which is given with reference to the figures in
which:

[0033] FIG. 1 depicts a schematic and partial view in cross section,
illustrating one example of a compressor impeller mounted on bearings and
comprising a device for collecting leaks of working gas according to the
invention,

[0034] FIG. 2 is a schematic and partial view illustrating the structure
and operation of a first embodiment of a refrigeration and/or
liquefaction device according to the invention,

[0035] FIG. 3 depicts a schematic and partial view illustrating the
structure and operation of a second embodiment of a refrigeration and/or
liquefaction device according to the invention,

[0036]FIG. 4 depicts a schematic and partial view illustrating the
structure and operation of a third embodiment of a refrigeration and/or
liquefaction device according to the invention,

[0037] FIG. 5 depicts a schematic and partial view illustrating the
structure and operation of a fourth embodiment of a refrigeration and/or
liquefaction device according to the invention.

[0038] The example of a refrigerator/liquefier that has been depicted in
FIG. 2 comprises, in the conventional way, a compression station 2 and a
cold box 3.

[0039] The refrigerator/liquefier uses a working fluid with a low molar
mass, and preferably predominantly or pure gaseous helium.

[0040] As depicted, this helium can be produced from a source S of
helium-rich gas, for example from natural gas (or some other gas) which
is purified by a purification unit 1 to supply helium to a working loop
of the refrigerator/liquefier. The purification unit or member 1
contains, for example, a cryogenic gas separation system and/or two
adsorbers arranged in parallel and operating in alternation on successive
adsorption/regeneration cycles (of the PSA or TSA type, for example). The
adsorbers are, for example, adsorbers of the active charcoal or
silica-based type to remove impurities such as air, nitrogen.

[0041] What that means to say is that the system forms an open loop with a
continuous influx of impurity.

[0042] In a conventional way, the working gas is compressed at ambient
temperature in the compression station 2 using one or several compression
stages 12 each using one or several compression machines, for example of
the centrifugal compression type.

[0043] Thus, at the inlet to the compression station 2, the working gas
arrives at a temperature close to ambient temperature and a pressure
known as the low pressure LP comprised for example between 1 and 3 bar
abs. At the outlet of the first compression stage 12, the working gas may
then reach a pressure known as the medium pressure MP comprised for
example between 3 and 8 bar abs.

[0044] At the outlet from the second compression stage 12, the working gas
may then reach a pressure known as the high pressure HP comprised, for
example, between 9 and 27 bar abs.

[0045] The compressed working gas is then admitted to a cold box 3 where
it is cooled (or pre-cooled). Conventionally, during this (pre)cooling,
energy (heat) is extracted from the working gas by expansion in one or
more cryogenic turbine(s) and/or by exchange of heat with a cryogenic
fluid such as nitrogen for example (for simplicity, the details of the
cold box have not been depicted).

[0046] Once the working fluid has exchanged heat with a user, it may then
return to the inlet of the compression station 2 (possibly being heated
back up gradually in exchangers).

[0047] As depicted schematically in FIG. 1, leaks of working gas (He)
occur particularly at the bearings 5 of the shaft 25 of the compression
impellers 12.

[0048] For preference, one or more sealing devices 15 are arranged around
the shaft 25 at each bearing 5, to limit the leaks of working gas
originating from the working circuit (sealing devices of the "labyrinth"
type, for example).

[0049] According to the invention, a barrier gas (nitrogen for example) is
injected into the bearings 5 and the shaft 25 in order notably to isolate
the working circuit from the mechanical part containing the oil O (gear
mechanism and motor(s) of the compression station). What that means to
say is that the barrier gas is designed to guide the leaks of working gas
towards an outlet 24. For example, two barrier gas N2 injection points 14
may flank an outlet path 24 for the mixture containing the injected
barrier gas N2 and the collected working gas He. For example, only the
barrier gas N2 passes through the part in contact with the oil O or the
atmosphere.

[0050] For preference, the pressure of the barrier gas injected is lower
than the pressure of the working gas at the compression impeller
concerned. In that way, any contamination of the working circuit by the
barrier gas is avoided.

[0051] It will additionally be noted that a leak of working gas into the
barrier gas is necessary for the correct functioning of this sealing zone
(notably for the purposes of cooling the bearings).

[0052] At the outlet 24, the barrier gas contains a not-negligible amount
of working gas (for example between 20 and 50 mol %). This mixture
(He+N2) therefore leaves the bearings at a relatively low pressure
comprised for example between 1 and 7 bar abs depending on the
compression stage concerned.

[0053] FIG. 2 depicts a first embodiment which may, for example, relate to
a liquefaction unit of an industrial type.

[0054] In this embodiment, the device comprises a gas purification member
1 having an inlet 11 for gas that is to be purified and an outlet 21 for
purified gas. The outlet 21 of the purification member 1 is fluidically
connected to the working circuit at the outlet of the compression station
2 or at a lower point in the cycle, dependent on its temperature.

[0055] The inlet 11 of the purification member 1 is fed with gas S from a
source, for example is fed with a mixture of methane, nitrogen and
helium. What that means to say is that the working gas in the working
circuit is fed in open loop with gas that is less pure and undergoes a
purification treatment.

[0056] In addition, the mixture of barrier gas and of working fluid
collected by the outlets 24 described hereinabove is reinjected, via a
pipe 13, at the inlet of the purification member 1 so that it can be
purified then reinjected into the working circuit at the outlet of the
compression station 2 or at a lower level in the cycle according to its
temperature. What that means to say is that the barrier gas mixed with
the working gas at the outlet 24 of the compressor system can be sent to
the intake side of the purification member 1, for example to the intake
side of a compressor that forms part of the purification member 1.
Specifically, this collected mixture contains a nitrogen impurity level
that may be compatible with operation of the purification member 1.

[0057] The working gas leakage rate is relatively low by comparison with
the flow rate through the compressor of the purification unit. The level
of sealing at each working gas compression stage 12 is therefore not
critical. As a result, expensive sealing solutions 15 at the bearings can
be avoided, in order to reduce the costs of the whole.

[0058] FIG. 3 depicts a second embodiment which may, for example, apply to
a refrigeration unit. In FIG. 3 et seq. the elements which are identical
to those described hereinabove are denoted by the same reference numerals
and not described again. The device in FIG. 3 operates in this embodiment
with a closed-loop working circuit (no supply of working gas via an
external source).

[0059] The mixture of barrier gas and of working fluid collected by the
outlets 24 is reinjected preferably directly at the inlet of the
compression station 2 or of an intermediate stage of the compression
station via a pipe 13. The mixture (barrier gas and working gas)
recovered at the outlets 24 of the compressors 2 is therefore injected
directly into the low-pressure circuit of the corresponding stage or of
the compression station.

[0060] This recycling may generate contamination (barrier gas such as
nitrogen) in the working circuit. These impurities are preferably removed
in the working circuit. This removal can be carried out either by
suitably rating the purification adsorbers conventionally provided in the
cold box 3 or by adding an additional purification system 1. Thus, as
depicted, the working circuit may optionally comprise a gas purification
member 1 that has an inlet 11 for gas that is to be purified fed with
working gas at the outlet of the compression station 2. The outlet 21 for
purified gas from the purification member 1 feeds the cold box 3.

[0061] In this type of refrigeration unit, the working cycle of which is a
closed loop cycle, losses of working gas at the bearings have to be
relatively limited.

[0062]FIG. 4 depicts a third embodiment which may, for example, relate to
a refrigeration unit. In this embodiment, the device operates with a
closed-loop working circuit. The mixture of barrier gas and of working
fluid collected by the outlets 24 is reinjected into the gas feed inlet
21 of a gas purification member 1. As before, the purification member 1
removes the impurities (removes all or some of the barrier gas, for
example using nitrogen adsorbers of the TSA or PSA type if the barrier
gas is nitrogen). What that means to say is that, in this case, the
mixture of barrier gas (nitrogen) and of working gas (helium) recovered
leaves the compressors 12 at a pressure that is high enough that it can
be admitted directly into a conventional low-pressure purification member
1.

[0063] The purified gas outlet 21 of the purification member 1 is
connected fluidically to the working circuit at the inlet of the
compression station 2. What that means to say is that the working fluid
is returned to the working circuit after purification.

[0064] FIG. 5 depicts a fourth embodiment which may, for example, relate
to a refrigeration unit. The device in this embodiment operates with a
closed-loop working circuit. The mixture of barrier gas and of working
fluid collected by all or some of the outlets 24 is reinjected into the
gas feed inlet 21 of a gas purification member 1 via a compressor 6. What
that means to say is that the mixture is compressed to a pressure which
is high enough to allow high-pressure or medium-pressure purification
thereof (pressures comprised for example between 3 and 27 bar abs). The
medium-pressure or high-pressure purified working gas is reinjected at an
intermediate compression stage of the compression station 2 and/or at the
outlet of the compression station 2.

[0065] Of course, the invention is not restricted to the exemplary
embodiments described hereinabove. For example, it is possible to
envisage a device that falls somewhere between the embodiments of FIGS. 4
and 5. What that means to say is that when the mixture recovered is at a
relatively low pressure, for example of between 1 and 3 bar (pressure in
the impellers of the first compression stage), this mixture can be
compressed to a medium pressure (of between 3 and 9 bar) before it is
purified or reinjected directly into the circuit. When the mixture
recovered is at a medium pressure, for example of between 3 and 15 bar
(pressure in the impellers of an intermediate compression stage), this
mixture can be sent to a medium-pressure purifier 1. In this way, the
size of the recuperation compressor can be reduced.

[0066] Likewise, it is possible to envisage a device that falls somewhere
between the embodiments of FIGS. 2 and 3. What that means to say is that
when the mixture recovered is at a relatively low pressure, for example
of between 1 and 3 bar (pressure in the impellers of the first
compression stage), this mixture can be reinjected directly into the
circuit at the inlet of the compression station.

[0067] In this case, in order to process the extra impurities, the
internal adsorbers which are commonplace in the cold box 3 (for purifying
working fluid) are preferably rated accordingly.

[0068] When the mixture recovered is at a medium pressure, for example of
between 3 and 15 bar (pressure in the impellers of an intermediate
compression stage), this mixture can be sent to a medium-pressure
purifier 1.

[0069] It will therefore be readily understood that, while a simple and
inexpensive structure, the invention allows any working fluid that has
leaked to be recovered and recycled.

[0070] The invention makes it possible to control the extent to which the
working gas is contaminated with a barrier gas. The working gas
contaminated with the barrier gas is recovered and purified (in the cold
box 3 and/or in an external purification member 1). This purification may
be carried out at medium pressure after compression (or after an increase
in pressure compatible with the sealing system). The purified gas can be
reinjected into the circuit at the low-pressure level and/or at the
medium-pressure level and/or at the high-pressure level.

[0071] The invention may notably be applied to any high-capacity
liquefaction or refrigeration unit (operating on a helium or rare gas
cycle).

[0072] The invention may notably also be applied to a hydrogen liquefier
using helium as the working gas.